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Review and Measurements of REL Wireless Subwoofer Transmitter

amirm

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#1
This is a review and detailed measurements of the REL Acoustics Wireless Subwoofer and Transmitter. I purchased it from Amazon and it costs US $98 including Prime shipping.

As the name indicates, this is a transmitter and associated receiver to allow you to locate the subwoofer for your home theater anywhere you wish. The pair of boxes are rather high quality:

REL Acoustics Ht-Air Wireless Transmitter and Receiver Audio Review.jpg

The external power supplies are small and provide 5 volt at 500 milliamps. As such, it could have used a USB charger as some of its competitors use (e.g. SVS). That way, you could power the transmitted by the USB port in your AVR.

There are a ton of these products in the market but from what I can tell, many are using Bluetooth. The REL does not and hence advertises low latency. This is good as while you can delay your main channels to match any delay, you may lose lip sync.

The two units paired automatically on power up although there is also a manual process for pairing which I believe could be used if you have more than one pair.

Audio Measurements
As usual, let's look at our dashboard but with the lower frequency tone of 30 Hz :

REL Acoustics Ht-Air Wireless Transmitter and Receiver Audio Measurements.png


There is some gain which was surprising to me.

SINAD is computed using the full audio bandwidth which doesn't apply here as your sub won't go that high. So in reality, performance is even better than 74 dB. This should be more than transparent for a sub as it is likely to have much worse distortions.

Frequency response shows that there is a built-in filter:
REL Acoustics Ht-Air Wireless Transmitter and Receiver Frequency Response Audio Measurements.png


This helps the response of the system as it is the higher frequencies which get noisier. The available bandwidth is plenty for any type of subwoofer you may use with it.

Let's see if the distortion varies during the typical subwoofer spectrum (and then some):

REL Acoustics Ht-Air Wireless Transmitter and Receiver THD vs Frequency  Audio Measurements.png


Not an issue. Yes, there is some rise in distortion but that is well above 100 Hz where the sub is likely rolling off anyway. And even there, we are talking -60 dB.

Testing level versus distortion we get:

REL Acoustics Ht-Air Wireless Transmitter and Receiver THD vs Level Audio Measurements.png


Other than clipping near 2 volts, the rest is pretty good actually given the fact that this is a wireless transmission.

I then ran a sweep of phase shift relative to frequency:

REL Acoustics Ht-Air Wireless Transmitter and Receiver phase delay Audio Measurements.png


Don't mind the sudden drops. The graph basically says the phase shift keeps increasing with frequency linearly which means it has a constant, frequency independent delay which is good.

So far so good.

RF Interference
I ran a real-time capture of THD+N:
REL Acoustics Ht-Air Wireless Transmitter and Receiver THD+N versus Time Audio Measurements.png


At around 42 seconds, my son turned on Wifi on his phone and ran a speedtest. As you see, that caused fairly high levels of THD+N, caused by sudden shifts in noise level and glitches. This was with him being 6 feet away from the transmitter. I then had him move 12 or so feet away and do the same thing. Strangely, the interference was worse there. RF works in magical ways! :) What the eye sees, is not in sync with how RF energy couples into something.

It is clear then that Wifi interference can cause serious issues with this device. I will be testing this in a scenario similar to this as my AVR is just a few feet away from my Wifi Access Point. Will report back on what the audible considerations might be later.

Conclusions
It took us three years to remodel our current home. I ran miles and miles of cables "just in case." And conduits. Alas, didn't think one day we may want to have more than 2 channels in our living room (thought we would use our dedicated theater a lot more than we are). So now I need a way to connect a subwoofer which would be sitting under a table in the middle of our living room. Hence the REL wireless subwoofer transmitter.

I am relieved to see the transparency that the REL achieves for this intended application. Interference with Wifi though is a concern. With just one spectrum (2.4 Gigahertz) available to every device known to man, this is kind of unavoidable. A more sophisticated spread spectrum system could have dealt with this better but likely not cheap to build. Bluetooth may do better given its error correction but without testing, it is hard to know.

Anyway, if you can keep this device away from Wifi, the REL will likely do the job. Recommendation pending actual listening tests.

------------
As always, questions, comments, recommendations, etc. are welcome.

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Vovgan

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#2
Thanks for the review! I have a REL t7i in my system and for the benefit of those considering purchasing a REL want to share two observations:

1) REL sounds fantastic. In stereo, it sounds especially good when connected via the high-level input directly to the speaker terminals on amp. Turning this sub on for a bass-rich track makes the bass sound wider and visceral on my B&W 703 floorstanders.

2) I have tried connecting it to my amp with the supplied 10m cable, and also with a REL's bassline cable ($450) and Van Damme cable for REL being sold for around $50 on eBay. The last two options sounded indistinguishable and clearly better than the supplied cable.

Have not tried this wireless option but good to know that it measures well. Looking forward to hearing Amir's opinion after doing the listening test.
 
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#4
@amirm In the interference test, what bandwidth did you use to measure the real-time THD+N? If WiFi causes mainly high-frequency noise, the problem may not be as big as it looks here.

Still, not a device i would like to use. Analog radio? No thanks. Digital, with error correction, i would consider.

I remember a story from a friend who used a similar solution, but his neighbor complained that when he used the system, he could no longer lock his car with his wireless key...
 

Soniclife

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#6
Regarding the wifi noise, is it broadband or confined to some part of the spectrum?
 

GrimSurfer

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#8
If these are the results of a test of a simple wireless transmitter for a subwoofer, I can imagine what might be seen with wireless full range speakers that rely on WiFi.

Channel overlap, multiple devices using the same channel, SMPS and microwave interference, time latenciesvwith repeaters/extenders etc. Are these problems resolvable? Yup. Are they likely to be resolved at low price points or if they are not grossly audible? Nope.
 

Soniclife

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#10
If these are the results of a test of a simple wireless transmitter for a subwoofer, I can imagine what might be seen with wireless full range speakers that rely on WiFi.

Channel overlap, multiple devices using the same channel, SMPS and microwave interference, time latenciesvwith repeaters/extenders etc. Are these problems resolvable? Yup. Are they likely to be resolved at low price points or if they are not grossly audible? Nope.
The chromecast audio managed it a very low price, so I don't see a problem if the designer is good.
 

speedy

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#11
Are you able to measure how much the delay is?
I've also wondered not just how much delay there is with these, but also how consistent the delay is. While dialing in a sub I usually spend the majority of my time with time/phase adjustments... I imagine that it would all be pointless if the timing of the sub weren't consistent.
 
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GrimSurfer

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#12
The chromecast audio managed it a very low price, so I don't see a problem if the designer is good.
I guess this makes sense given that chromecast development would have been overseen or conducted on behalf of Google, a web centric company. Agree that shielding against RF interference in the GHz range isn't all that difficult, especially at the levels of radiated power we're talking about.

Amir's findings again expose another blind side of a respected audio company.
 

amirm

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#13
Regarding the wifi noise, is it broadband or confined to some part of the spectrum?
The device itself limits to 1 kHz so it is low frequency in nature. I also filtered the heck out of everything above 200 Hz:

1569609093615.png


So the interference will definitely happen to the spectrum fed to the subwoofer.

The Wifi itself is high frequency but so is the carrier for this device. Once its carrier gets swamped by noise, then demodulating and extracting the data becomes hard, resulting in loss of data/signal.
 
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#14
I guess this makes sense given that chromecast development would have been overseen or conducted on behalf of Google, a web centric company. Agree that shielding against RF interference in the GHz range isn't all that difficult, especially at the levels of radiated power we're talking about.

Amir's findings again expose another blind side of a respected audio company.
The way I understand it is that this device uses wifi to send an analogue signal through wifi. In case of the chromecast or raspberry pi based streamer a digital signal is sent through wifi.
I would expect wifi interference with a digital signal to result in glitches, ticks etc?
 

vkvedam

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#15
The way I understand it is that this device uses wifi to send an analogue signal through wifi. In case of the chromecast or raspberry pi based streamer a digital signal is sent through wifi.
I would expect wifi interference with a digital signal to result in glitches, ticks etc?
There is no way to send any signal using WiFi without the digitisation process.
 

amirm

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#17
Just tested it in our living room. No amount of messing with Wifi would impact the sound of the movies we tested it with. Don't have a good way to run tones through it right now but as it is, it works perfectly even at 20 feet or so.
 

amirm

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#18
Can I use a pair of these it instead of a tube buffer on the output of my DAC?
Sure. You could get the same effect by putting a blanket in front of your speaker though. :)
 

amirm

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#20
So this is my ignorance then. The signal is pcm encoded in the sender and the receiver has a dac that decodes it? You cannot send a fm type signal through wifi?
We don't know what the architecture is. It may be purely analog using the same spectrum as Wifi as that is one of the few unlicensed spectrums available for this use. Using the same frequency (2.5 Gigahertz) doesn't make it Wifi.
 
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